50th International Conference on Environmental Systems ICES-2021-134 12-15 July 2021 ExoMars Rover Module: Verification of the Loop Heat Pipes thermal performance in system level testing V. Laneve1 ESA ESTEC / RHEA System B.V., Noordwijk, 2201 AZ, The Netherlands M. Munì2 and L. Tentoni3 Thales Alenia Space, Torino, 10146, Italy and L. Tamkin4 Airbus Defence and Space Ltd., Stevenage, SG12AS, United Kingdom The ExoMars program is a joint endeavor between ESA and Roscosmos. It includes the Trace Gas Orbiter launched in 2016 and the Rover and Surface Platform mission, whose launch has been recently rescheduled for 2022. The ExoMars Rover Module (developed by Airbus Defence and Space and Thales Alenia Space) underwent its system level test campaigns in 2018 and 2019. One of the key features of the Rover Module thermal design is the use of Loop Heat Pipes acting as heat switches to insulate the rover internal equipment and payloads during the cold Martian night and rejecting the heat dissipated by the electronics units when the rover is operating during the day. The ExoMars Rover Module is equipped with four Loop Heat Pipes, two of them integrated in the Service Module (SVM) and the other two on the Analytical Laboratory Drawer (ALD). Each Loop Heat Pipe is designed to transport up to 50 W and operate at temperatures from -50°C to +55°C at the evaporator and from -120°C to +55°C at the condenser. The vapor modulation and switching function is achieved by means of Pressure Regulating Valves. This paper will provide an overview of the activities performed at system level to verify the functional performance of the Rover Module Loop Heat Pipes and their main outcomes. Nomenclature AA = Aluminium Alloy AIT = Assembly, Integration and Testing ALD = Analytical Laboratory Drawer CEU = Control Electronic Unit CO2 = Carbon Dioxide DM = Descent Module FPGA = Field Programmable Gate Array GN2 = Gaseous Nitrogen IMU = Inertial Measurement Unit ISEM = Infrared Spectrometer for ExoMars LHP = Loop Heat Pipe MOMA = Mars Organic Molecule Analyzer OBC = On-Board Computer PCDE = Power Conditioning and Distribution Electronics 1 ExoMars RSP Thermal Systems Engineer, TEC-MTT, [email protected] 2 ExoMars RM Thermal Systems Engineer, Thermal Systems, [email protected] 3 Thermal Engineer, Thermal Engineering, [email protected] 4 MSR-SFR STHS Development Manager and Thermal Lead, Thermal Engineering, [email protected] Copyright © 2021 European Space Agency PFM = Proto-Flight Model PRV = Pressure Regulating Valve RHU = Radioisotope Heating Unit RLS = Raman Laser Spectrometer RM = Rover Module RSP = Rover and Surface Platform SPDS = Sample Preparation and Distribution System SS = Stainless Steel STM = Structural and Thermal Model SVM = Service Module TCS = Thermal Control Sub-system TVAC = Thermal-Vacuum UCZ = Ultra-Clean Zone WISDOM = Water Ice and Subsurface Deposit Observation on Mars I. Introduction HE ExoMars program is a joint endeavor between ESA and Roscosmos. In addition to the Rover and Surface T Platform (RSP), it also includes the Trace Gas Orbiter launched in 2016. A Proton rocket will be used to launch the RSP mission, which is planned to arrive at Mars after a nine month journey. The Rover Module (developed by Airbus Defence and Space and Thales Alenia Space) will travel across the Martian surface to search for signs of life. It will collect samples with a Drill (developed by Leonardo) and analyse them on its on-board laboratory (ALD). A total of nine scientific payloads are hosted on the Rover Module. They are provided by ESA member states, NASA and IKI/Roscosmos. The Surface Platform, developed by Roscosmos and IKI, will remain stationary and investigate the surface environment at the landing site. The Rover Module underwent its system level thermal testing in 2018 (STM) and 2019 (PFM) at Airbus Defence and Space test facilities in Toulouse. II. Thermal Design of the ExoMars Rover The thermal design of the ExoMars Rover Module was discussed in the past in Ref. 1. However, during the development of the project, some modifications were introduced in order to improve the performance of the rover. Therefore, a brief overview of the main features of the RM thermal design is provided in this section. The RM thermal design is characterized by three distinct thermal zones that are controlled independently (see Figure 1 and Figure 2): 1) SVM zone: the area inside the rover body where the warm electronics are accommodated. It is thermally isolated from the external environment and the ALD. Two units (the ADRON and ISEM payloads) lie outwith the SVM zone but are conductively connected to the main SVM panel with thermal straps. 2) ALD zone: the area inside the rover body where the three analytical instruments (MOMA, RLS and MicrOmega) are located. 3) External zone: the area outside the main rover body that is subjected to the full swing of environmental conditions on Mars. The volume inside the rover body is thermally insulated from the external volume and is thermally split in two by a barrier through the center of the structure to form the zones described above. The thermal decoupling between the rover internal volume and the external environment relies on the insulation provided by the low pressure CO2 atmosphere on Mars (gas-gap concept with typical size of 20mm to 30mm achieved by means of inner baffles2,3), the use of optical finishes with low IR emissivity, isostatic mounts with low thermal conductance and LHPs. The LHPs are used as heat switches to assure a high thermal de-coupling between internal zones (SVM, ALD) and cold external environment during the Martian night and to remove the heat generated by the electronic units during the day, thus allowing normal operations such as traversing and scientific experiments. Furthermore, RHUs and heaters are installed to maintain the temperature of the SVM and ALD within the allowable range in the cold phases of the surface mission. The SVM hosts within its insulated space, the Battery, the Transceivers, the IMUs, the OBC, the PCDE, the WISDOM electronic unit and two RHUs (each of 8.5±0.5 W dissipation). Among the units accommodated on the SVM panel, the Battery has the most stringent requirements for thermal control and has driven the selection of the control set-point of the SVM LHPs (i.e. 0°C). The support panel in the center of the SVM equipment is designed to 2 International Conference on Environmental Systems provide both mechanical support and thermal coupling. As such the RHUs and LHPs are mounted onto this panel in order to ensure an adequate heat spreading or rejection during a sol. In addition to MOMA, RLS and MicrOmega, the ALD hosts the SPDS and the CEU and provides the necessary structural support and thermal control functions. A contamination controlled volume, called Ultra Clean Zone (UCZ), is located on the lower panel. The processing of the samples extracted by the Drill and the specimen’s preparation, as well as the “life-detection” analyses, are performed within the UCZ. The ALD is equipped with one RHU, two LHPs, survival and warm-up heaters. Figure 1. Overview of the Rover Module internal layout: top view without Solar Array (left) and cross- sectional view in stowed configuration (right). Figure 2. Overview of the Rover Module external layout in deployed configuration 3 International Conference on Environmental Systems Figure 3. SVM (Left) and ALD (Right) accommodation III. The ExoMars Rover Loop Heat Pipes The RM is equipped with four Loop Heat Pipes, two of them integrated in the SVM and the other two in the ALD. Each Loop Heat Pipe is designed to transport up to 50 W and operate at temperatures from -50°C to +55°C at the evaporator and from -120°C to +55°C at the condenser. As a consequence of the temperatures expected on Mars, propylene has been selected as working fluid. In the RM PFM the radiators are white-painted with inorganic coating (AZ2100-IECW). A summary of the main design features of the LHPs is presented in Table 1. The vapor modulation and switching function is achieved by means of Pressure Regulating Valves (PRVs). The PRV operation is driven by the movement of a piston which depends on the pressure balance between the working fluid in the LHP and in the PRV reservoir, filled with argon. The valve operation is fully passive: the working fluid pressure is related to the LHP operating temperature (saturation curve), whereas the argon (gas) pressure variation is much lower within the same temperature range. The difference in pressure between the two fluids leads to the piston movement. The LHP has basically three operational modes. In the first mode (“ON mode”), the PRV is fully open. The pressure of the working fluid in the LHP is higher than the pressure of the argon plus the piston load, therefore the LHP can be considered as a constant conductance device. In the second mode (“regulation mode”), the PRV piston moves closer to the fully closed position. In this situation, there is still fluid circulation in the LHP, but the PRV position causes an additional pressure drop, which is reflected in a decrease of the thermal conductance of the LHP. Finally, the third operational mode corresponds to the “OFF mode”, in which the PRV is fully closed and therefore no fluid circulation exists in the LHP. The thermal conductance in this case corresponds only to the conduction through the LHP transport lines. Two different set-points have been chosen for the ALD and SVM PRVs, namely: -40°C for the ALD (to assure the preservation of the Martian soil samples) and 0°C for the SVM (mainly driven by the Battery allowable temperature range).